This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Pharmacokinetics of levothyroxine (LT4) will be conducted in supplemented hypothyroid non-pregnant and pregnant women. Adequate thyroxine is critical for fetal brain development. Hypothyroid women need to be supplemented adequately especially during pregnancy due to normal increased levels of thyroxine during pregnancy. We will examine LT4 PK in pre-pregnant (or post partum) and pregnant women (1st trimester).